Ovarian follicular growth was monitored by ultrasound and plasma estradiol levels in 43 cycles of 27 patients. These women were treated with human menopausal gonadotropin (hMG) and human chorionic gonadotropin (hCG) for anovulation or poor cervical mucus. Ultrasonography can help to prevent multiple pregnancies by allowing the withholding of the ovulatory injection of hCG if the number of mature follicles is too great. Nevertheless, mild to moderate hyperstimulation cannot always be prevented; it is sometimes the price one has to pay to obtain a pregnancy. Giving a direct view of follicular maturation, this method can be helpful in the determination of the best time for the induction of ovulation.